1. Field of the Invention
Aspects of the present invention relate to a polymer electrolyte membrane for a fuel cell, a method of manufacturing the same, and a fuel cell employing the same. More particularly, aspects of the present invention relate to a polymer electrolyte membrane having excellent durability and that is adapted for a fuel cell that is operated at a temperature range of 100 to 200° C. without using a particular fuel gas humidifier, a method of manufacturing the same, and a fuel cell employing the same.
2. Description of the Related Art
Polymer electrolyte membranes used in solid polymer fuel cells require high ionic conductivity, and at the same time, good long-term stability. U.S. Pat. No. 5,525,436 discloses an electrolyte membrane in which a polybenzimidazole film is doped with an orthophosphoric acid. Japanese Patent Laid-Open Publication No. 2000-281819 discloses a polymer electrolyte membrane having an improved mechanical strength, in which polybenzimidazoles are crosslinked by diepoxy compounds or the like. Japanese Patent Publication No. 2005-535734 discloses an electrolyte membrane in which a polymer blend including polybenzimidazoles is crosslinked by diepoxy compounds or the like. In summary, U.S. Pat. No. 5,525,436, Japanese Patent Laid-Open Publication No. 2000-281819, and Japanese Patent Publication No. 2005-535734 relate to electrolyte membranes in which polybenzimidazoles, which are basic polymers, are impregnated with a dopant (e.g., an orthophosphoric acid). Membrane and electrode assemblies employing these electrolyte membranes tend to be unsatisfactory in view of long-term stability. The electrolyte membranes disclosed in Japanese Patent Laid-Open Publication No. 2000-281819 and Japanese Patent Publication No. 2005-535734 have an improved mechanical strength provided by adding a crosslinking agent, but the long term stability of these membranes nevertheless tends to be insufficient. The cause of the lack of long term stability is that these kinds of electrolyte membranes undergo a large dimensional change between the wet state (moisture absorption state) and the dry state.
That is, an electrolyte membrane that includes a large amount of an acidic dopant exists in a wet state due to product water when a fuel cell containing the electrolyte membrane is in a closed circuit state, but the electrolyte membrane exists in a dry state due to leakage of water from the electrolyte membrane when the fuel cell is in an open circuit state or is operating at a low current density.
Due to absorption and desorption of water between the wet state and dry state of an electrolyte membrane, the electrolyte membrane undergoes a dimensional change. In particular, an electrolyte membrane, the edges of which are fixed in a cell, is subjected to tensile stress due to a dimensional change in the planar direction. In order to solve this problem, a method of enhancing the yield strength or fracture strength of an electrolyte membrane has been proposed, but there is much room for improvement.